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Chapter 32

BISC300 Chapter 32: Chapter 32 Notes

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Biological Sciences
Cooper Carlton

Chapter 32 Notes: Microbial Infections ➢ 32.1 Many Types of Microbial Interactions Exist The stable association of one organism with another is called symbiosis, regardless of whether the association is beneficial for both partners ▪ Symbiosis: The living together or close association of two dissimilar organisms, each of these organisms being known as a symbiont. Symbionts can live within the host as an endosymbiont, or on the surface of the host as an ectosymbiont. Finally, symbionts alter not only the health of their hosts, but their behavior, evolution, and reproductive success as well Mutualism defines the relationship in which some reciprocal benefit accrues to both partners. This is an obligatory relationship in which the mutualist and the host are dependent on each other. In many cases, the individual organisms will not survive when separated. Several examples of mutualism are presented next. ▪ Mutualism: A type of symbiosis in which both partners gain from the association and are dependent on each other. ▪ Mutualist: An organism associated with another in an obligatory relationship that is beneficial to both. Many marine invertebrates (sponges, jellyfish, sea anemones, corals, ciliated protists) harbor endosymbiotic dinoflagellates called zooxanthellae within their tissues. ▪ Coral bleaching: The loss of photosynthetic pigments by either physiological inhibition or expulsion of the coral photosynthetic endosymbiont, zooxanthellae, a dinoflagellate. Rumen Ecosystem Ruminants are the most successful and diverse group of mammals on Earth today. Examples include cattle, deer, elk, bison, water buffalo, camels, sheep, goats, giraffes, and caribou. These animals spend vast amounts of time chewing their cud—a small ball of partially digested grasses that the animal has consumed but not yet completely digested. It is thought that ruminants evolved an “eat now, digest later” strategy because their grazing can often be interrupted by predator attacks. When the animal eats plant material, it is mixed with saliva and swallowed without chewing to enter the rumen. Here, microbial attack and further mixing coats the grass with microbes, reducing it to a pulpy, partially digested mass. At this point, the mass moves into the reticulum, where it is regurgitated as cud, chewed, and reswallowed by the animal. As this process proceeds, the grass becomes progressively more liquefied and flows out of the rumen into the omasum and then the abomasum. Here the nutrient-enriched grass meets the animal’s digestive enzymes, and soluble organic and fatty acids are absorbed into the animal’s bloodstream. The oxidation of these organic substrates, which results in the generation of H2 quickly consumed by methanogens, is termed syntrophy. Syntrophy is an association in which the growth of one organism either depends on, or is improved by, growth factors, nutrients, or substrates provided by another organism growing nearby. ▪ Syntrophism: The association in which the growth of one organism either depends on, or is improved by, the provision of one or more growth factors or nutrients by a neighboring organism. Sometimes both organisms benefit. For most microbial ecologists, the nonobligatory aspect between host and symbiont differentiates cooperation from mutualism. ▪ Cooperation: A positive but not obligatory interaction between two different organisms. Commensalism ▪ Commensalism: A type of symbiosis in which one individual gains from the association (the commensal) and the other is neither harmed nor benefited. Commensalism is a relationship in which one symbiont, the commensal, benefits, while the host is neither harmed nor helped. Commensal is the partner who benefits 2 Figure 32.9 Examples of Predatory Bacteria Found in Nature. (a) Vampirococcus has a unique epibiotic mode of attacking a prey bacterium; and (b) Daptobacter showing its cytoplasmic location as it attacks a susceptible bacterium. Parasitism: Microbes Harm the Host Parasitism is one of the most complex microbial interactions; the line between parasitism and predation is difficult to define. In both cases, the relationship between two organisms is beneficial for one and harmful for the other. However, parasite and host must coexist, at least temporarily, so the microbe has enough time to reproduce and colonize a new host. Coexistence may involve nutrient acquisition, physical maintenance in or on the host, or both. But what happens if the host-parasite equilibrium is upset? If the balance favors the host (perhaps by a strong immune defense or antimicrobial therapy), the parasite loses its habitat and may be unable to survive. On the other hand, if the equilibrium is shifted to favor the parasite, the host becomes ill and, depending on the specific host-parasite relationship, may die. ▪ Parasitism: A type of symbiosis in which one organism benefits from the other and the host is usually harmed. ▪ Parasite: An organism that lives on or within another organism (the host) and benefits from the association while harming its host. Often the parasite obtains nutrients from the host. Genomic Reduction occurs when the symbiont has become dependent on the host for specific functions, such as synthesis of key metabolites. It is observed in the aphid endosymbiont Buchnera aphidicola; it has also occurred in the human pathogens Mycobacterium leprae, 3 Mycoplasma genitalia, and Encephalitozoon cuniculi, a microsporidium. These microorganisms now can only survive inside their host cells. ▪ Genomic Reduction: The decrease in genomic information that occurs over evolutionary time as an organism or organelle becomes increasingly dependent on another cell or a host organism. Competition arises when different organisms within a population or community try to acquire the same resource, whether this is a physical location or a particular limiting nutrient. If one of the two competing organisms can dominate the environment, whether by occupying the physical habitat or by consuming a limiting nutrient, it will overtake the other organism. This phenomenon was studied by E. F. Gause, who in 1934 described it as the competitive exclusion principle ▪ Competition: An interaction between two organisms attempting to use the same resource (nutrients, space, etc.). ▪ Competitive exclusion principle: Two competing organisms overlap in resource
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